Add initial code

audio/GameStartMusicTween.gd

@@ -0,0 +1,14 @@
+class_name GameStartMusicTween
+extends Tween
+
+# https://godotengine.org/qa/27939/how-to-fade-in-out-an-audio-stream
+
+func fade_music_out(stream_player: AudioStreamPlayer):
+	# tween music volume down to 0
+	self.interpolate_property(stream_player, "volume_db", stream_player.volume_db, -80, 4, 1, Tween.EASE_IN, 0)
+	self.start()
+
+
+func _on_GameStartMusicTween_tween_completed(object, key):
+	# stop the music -- otherwise it continues to run at silent volume
+	object.stop()